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'Offline' Venture Cx clock isn't as accurate as I thought


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I noticed that when my VentureCx had not been used for a few days and I switched it on (indoors so it doesn't get a chance to 'synchronize' with any satellites) the time was wrong by 'tens' of seconds.

 

So I did a test and I found that the GPSr was fast by 5 sec a day. I was a bit disappointed with that, as a cheap quartz watch would have much better accuracy than that.

 

It was as I was trying to synchronize my digital camera and my GPSr clocks so that I could get very precise geotagged photos that I noticed this 'quirk'. After I had gone outside and the GPSr clock had 'synchronized' with the satellites I realized my digital cameras clock was now well out of sync with the GPSr.

 

I am presuming (hoping!) that my GPSr isn't broken. I wonder, did anyone else notice this clock inaccuracy?

 

Once the GPSr gets a fix on the satellites the clock does get corrected automatically.

Edited by passdump
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Magellens seem to have a big problem with clocks. Garmin do ok. First of all the display clock is just that. It is not an atomic clock. Niether does the interior of the gps have an atomic clock. However, the GPS needs to have the accuracy of an atomic clock to give proper postion. They do this by syncronizing patterns in sat signals that transmit the product of the atomic clocks on the sat. If the signal is off by 1 nanosecond your position will be off by 1 foot.

I always assumed that the GPS sync the display clock fairly often to give it a degree of accuracy. After all, that IS what a GPS really is! A time transfer device. I recently wondered about my 60cx so I checked it against the time counter on short wave and it was dead on. My old garmin 12 however was a half second slow.

Modern GPS's should have the ability to be dead accurate on the display clocks once the sat almanacs are downloaded on start up. If not, we should really holler at the manufacturers!!

Ask them what the "H" they think a GPS "IS"? It is primarily "timing"! Just look at any cell tower and count the GPS antennas that do not move. It is navagation secondly.

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Now you have seen one of the reasons that handheld GPS units are not that accurate. GPS is fundamentally a system for measuring distance by knowing the velocity of electromagnetic wave propagation and the time. The clocks on the GPS satellites are very acccurate atomic clocks, and therefore very expensive. The clocks in most receivers are very inexpensive and inaccurate. With a big improvement in the clock accuracy of a handheld unit the positional accuracy would vastly improve a lot. Unfortunately there are no small, very accurate clocks that are currently available at prices which would allow them to be added into handheld units.

 

This is the main reason that you need to acquire four satellites to get a fix instead of three. The four times are used to make timing corrections.

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Yes, the internal clocks in a reciever are inexpensive, and yet I think GPSr's are quite accurate. Understanding that one nanosecond of error equals one foot of error, GPS's would not function unless they had the near accuracy of atomic clocks. So how can they do this with cheap clocks? As I understand it, by matching up known carrier codes for the sats with the recieved carrier codes. Sliding them together until they match.

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Most error in a GPS is due to signal delay as the signal passes through an active ionosphere, sat clock and position error due to relativity, and multipathing. WAAS and L2 band helps much of this.

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Just to clarify, once the GPSr is locked on to the satellites the display clock is dead accurate.Its perfect.

 

The issue I wanted to highlight was that from the time the GPSr goes 'offline' the accuracy is worse than a 'cheap and nasty' quartz watch. Surprising, really, considering how inexpensive, ubiquitous and yet very accurate quartz movements are. I know the Venture Cx is a relatively inexpensive GPSr but when you look at what it does it is really quite an astonishing little modern 'miracle' -all GPSrs are. From my layman's view synchronizing with 12 satellites, piloting a fix etc. are the difficult bits that the GPSr does, I would have thought keeping accurate time would have been easy!

 

It came to mind that just as a desktop or laptop's clock is also inaccurate (if not let synchronize through the internet) maybe the GPSr also uses a similar type of timing system (as opposed to a quartz based clock)? May be that's got something to do with it.

 

 

Yes, the internal clocks in a reciever are inexpensive, and yet I think GPSr's are quite accurate. Understanding that one nanosecond of error equals one foot of error, GPS's would not function unless they had the near accuracy of atomic clocks. So how can they do this with cheap clocks?

 

I agree to a point. As long as the unit is locked on to the satellites, I believe you have as near as you can get to a hand held atomic clock.However, I don't believe this is due to the accuracy of the internal GPSr clock, I think it is more to do with the fact it is 'networked' to the satellite's atomic clock, and is constantly being updated to the satelites clock. Once the GPSr is not allowed to synchronize with satellites for any length the GPSr's clock (relatively) quickly goes wrong.

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The usual quartz watch goes off by 30 seconds a month, which is the best you can get without paying several thousands of dollars.

 

Now as for the computer clock, it seems (after google research) that it would also be a quartz clock. Maybe it is the same for GPSr's... Anyone had to change his GPS internal battery?

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